Cytochrome bc1 and b6f complexes act in energy-transducing membranes to create a proton gradient that is used for the synthesis of ATP. The energy required to establish this- gradient comes from an oxidation- reduction process in which a quinol is oxidized and a high-potential electron acceptor is reduced. All cytochrome complexes contain three electron transfer proteins: a b-type cytochrome, a c-type cytochrome and a high potential 2Fe-2S Rieske Fe-S protein. This project focuses on the structure and function of several of these electron transfer proteins in reactions catalyzed by the cytochrome b6f complex as well as considering possible roles for other protein subunits that have recently been found in this complex. The approach taken in the project is to employ a combination of molecular biological techniques in conjunction with biochemical and biophysical analyses. Thus, the use of cloned genes for cytochrome f and the Rieske Fe-S protein will allow for site-directed mutagenesis of amino acids proposed to play specific roles in the function and structure of these proteins. The use of the green alga, Chlamydomonas, will facilitate these experiments since this organism can be grown non-photosynthetically to maintain mutants and is transformable. In addition to these studies, an analysis of cytochrome complexes from the green sulfur bacterium, Chlorobium, and the heterotrophic bacterium, Bacillus, will be carried out in order to define if these organisms contain cytochrome bc1 or b6f-type complexes. These studies will provide detailed evidence on the role of the Rieske iron-sulfur protein and other electron carriers in the catalytic reactions of the cytochrome b6f complex.